The apical sodium-dependent bile acid transporter (ASBT) is the major carrier protein involved in intestinal reclamation of bile salts. Complete genetic disruption of its activity leads to pathologic bile acid induced diarrhea in humans, while partial inhibition can be used to treat hypercholesterolemia and cholestasis. ASBT is expressed on the apical surface of ileal enterocytes, renal tubules, and cholangiocytes. In the rat ileum, ASBT undergoes a biphasic pattern of developmental expression. Bile acid responsiveness of ASBT is species specific. ASBT is down regulated in the ileitis and up regulated by corticosteroids. Adaptation by ASBT after intestinal resection is dependent upon the length of residual ileum. These changes in ASBT expression are controlled at the level of transcription and mRNA stability. The rat, mouse, and human ASBT promoters have been cloned in my laboratory. HNF-1alpha, AP-1 and LRH-1 are critical elements in the transcriptional regulation of ASBT. This proposal will define the molecular mechanisms involved in the regulation of the ASBT gene. Transcriptional regulation will be studied with chimeric luciferase promoter reporter constructs, DNA:protein gel shift analysis, and transgenic mice. Molecular mechanisms of transcriptional modulation of ASBT gene expression will be analyzed in normal physiology and pathologic states (altered bile acid homeostasis, ileal inflammation and intestinal resection). The role of mRNA stability will be assessed by identification of mRNA destabilizing elements in the ASBT transcript and by characterization of RNA:protein interactions. Cell-free systems and/or transgenic animals will be utilized to assess the role of these elements in developmental-stage and organ specific expression. The results of these studies will be highly significant in light of the crucial role that ASBT plays in human health and disease. The ASBT gene also represents a valuable model of important and poorly defined biological processes in the intestine, such as transcriptional regulation of ontogeny and tissue specificity and the role of mRNA stability in intestinal gene regulation.